Gas turbine engines, such as those used to power modern aircraft or in industrial applications, include a compressor for pressurizing a supply of air, a combustor for burning a hydrocarbon fuel in the presence of the pressurized air, and a turbine for extracting energy from the resultant combustion gases. Generally, the compressor, combustor and turbine are disposed about a central engine axis with the compressor disposed axially upstream of the combustor and the turbine disposed axially downstream of the combustor. Air drawn into the engine passes axially through the compressor into the combustor wherein fuel is combusted in the air to generate and accelerate combustion gases that pass through the turbine and out the exhaust nozzle of the gas turbine engine. The combustion gases turn the turbine, which turns a shaft in common with the compressor to drive the compressor.
The turbine of the gas turbine engine is generally an axially extending assembly of a plurality of turbine modules mounted to a shaft. Each turbine module may include one or more turbine stages. Each turbine stage includes a row of stationary airfoils, referred to as the stator vanes, and a row of airfoils, referred to as rotor blades, mounted on a rotor disk driven by the airflow passing over the rotor blades. The turbine may include a high pressure turbine including a plurality of high pressure stages in one or more modules assembled to a common shaft with a high pressure compressor, as well as a low pressure turbine including a plurality of low pressure stages in one or more modules assembled to a common shaft with a low pressure compressor and/or fan.
As the hot combustion gases pass through the turbine, various turbine elements are exposed to the hot combustion gases, which may also be corrosive to the material of which those turbine elements are made. Typically, the turbine vanes and turbine blades are made of materials capable of extended operation at high temperatures, such as nickel-based superalloys. In order to protect these turbine elements from oxidation and corrosion due to exposure to the hot combustion gases, it is conventional practice to coat various turbine elements with one or more layers of a protective coating or coatings.
For example, it is known to coat turbine blades for use in gas turbine engines with a protective coating during the process of manufacturing the turbine blades. A difficulty often encountered in the process of coating turbine blades arises from the requirement to coat different portions of the turbine airfoil with different thicknesses of the protective coating and/or with different types of protective coating. Thus, when depositing a protective coating on a selected portion of the turbine blade, it is desirable to protect the remaining portion or portions of the turbine blade from deposition of the protective coating.
One method of protecting a portion of the turbine blade from the deposition of coating material is to mask that portion of the turbine blade with masking tapes, slurries or other coatings. However, doing so is labor-intensive and time consuming. Another method of protecting a portion of the turbine blade from the deposition of coating material is to place the portions of the turbine blade to be protected into a masking enclosure, while leaving only that portion of the turbine blade to be coated exposed to the coating deposition process.
Conventionally, turbine blades include a blade platform, an airfoil portion extending outwardly from an upper face of the platform and a dovetail portion extending outwardly from an under face of the platform. It is customary practice to deposit a desired thickness of a desired protective coating material on the under surface of the blade platform and on the adjacent area of the shank end of the dovetail adjoining the under face of the blade platform. To protect the remainder of the turbine blade during the coating process, the blade is typically placed within a masking enclosure that generally effectively precludes deposition of the coating on the airfoil and dovetail of the blade. The assembly is subjected to a coating process wherein a vapor phase coating, for example an aluminum coating, is deposited only on the under face of the blade platform and the adjacent area of the shank portion of the blade dovetail, which are exposed to the vapor phase coating through one or more openings in the masking enclosure.
In conventional practice, such masking apparatuses for coating application are fabricated by welding metal stock to form the desired geometry for enclosing the turbine blade. However, distortion of the metal stock is inherent in the welding process, making it difficult to achieve the desired the level of accuracy needed to meet engineering requirements for these types of tools. Another shortcoming of fabricating a masking apparatus by welding is the lack of repeatability between such masking apparatuses. As a consequence, the specified coating requirement results may not be achieved, resulting in parts being stripped and recoated till they meet the specified requirements. There is also the risk of parts incorrectly coated to make it by the inspection process since it is difficult to completely determine the coating thickness across a part's surface. Additionally, in such masking apparatuses, the blade is not securely positioned within the masking enclosure. Therefore, the blade may undesirably move laterally within the masking enclosure.